Field of the Invention
Embodiments of the present invention generally relate nuclear imaging and more specifically to systems, methods, apparatuses, and computer-readable mediums for determining the orientation/location of immobile detectors/detector electronic assemblies.
Description of the Related Art
In PET imaging, for example, positrons are emitted from a radio-pharmaceutically doped organ or tissue mass of interest. The positrons combine with electrons and are annihilated and, in general, two gamma photons which travel in diametrically opposite directions are generated simultaneously upon that annihilation. Opposing crystal detectors, which each scintillate upon being struck by a gamma photon, are used to detect the emitted gamma photons. By identifying the location of each of two essentially simultaneous gamma interactions as evidenced by two essentially simultaneous gamma emissions from a positron annihilation event, a line in space along which the two gamma photons have traveled (a “line of response,” or “LOR”) can be determined, from which the location of the original positron annihilation event can be calculated. The LORs associated with many million annihilation events with the detectors are calculated and “composited” to generate an image of the organ or tissue mass of interest, as is known in the art.
Conventionally, an array of PET crystal detectors may be arranged circumferentially all the way around a bed on which the patient lies during the scan, with the bed oriented horizontally and the “ring” of detectors oriented in a vertical plane with the bed extending axially through the center of the ring. In such a case, with detectors completely surrounding the patient bed, the detectors remain stationary. (The bed may move longitudinally to image different regions of interest of the patient's body).
There are scanning systems where detectors and detector electronic assemblies (“DEAs”) rotate on a gantry, other scanning systems where the detectors and DEAs move intermittently, and yet other systems where the detectors and DEAs remain immobile.
In systems where the detectors remain immobile (i.e., stationary and not intended to move), it is still necessary to know the position in space of each detector (i.e., by digitally “tagging” or identifying each PET interaction event with its associated detector position) so that the LORs can be constructed.
Even though the detectors remain stationary, there are times when the scanner system is relocated to a different location and the spatial location of a detector(s) is changed as a result. There are also times when detectors are replaced. These movements result in the detectors/DEAs having a possibly unknown angular orientation and location.
In general, the detector position can be determined if the angular orientation in space of the detector is known, since each detector around the ring of detectors will have a unique angular orientation. Current schemes set in hardware—usually by use of DIP switches—the circumferential position of each of the acquiring detector's electronics, thereby providing a basis by means of which individual detector pixels may be encoded. DIP switches are used to determine a detector electronic assembly (“DEA”) location with respect to other detector electronic assemblies and a patient bed. The DIP switches are located on a circuit board of the DEA. DIP switches, however, may require manual setting and can be difficult to access. In addition, DIP switches require time to set and can be easily set to an incorrect setting.
Other PET imaging systems, on the other hand, use fewer detectors, and the detectors do not completely encircle the patient bed. For example, PET systems are known which use just two opposing detectors that are supported by a gantry, and the detectors are rotated by the gantry, e.g., through 180° each, so as to acquire a full 360° sweep of the patient. Other types of imaging systems such as SPECT imaging systems, as well as others, may use even less detectors, i.e., a single detector, and also acquire a fully swept image by rotating the detector around the patient, e.g., through a full 360°.
These non-fully-encircling systems (PET, SPECT, and others) also rely on knowing the position of the detector in space in order to construct LORs or otherwise generate an image of the patient. In such rotating systems, the detector position in space is typically determined by determining the rotational position of the gantry, which requires geared linkages and/or encoders. “Play” between system components can, however, cause inaccuracies in the detector positions determined by such means.
Accordingly, improved instrumentalities for determining the position of nuclear imaging detector(s) and DEAs, in a system in which the detectors and DEAs remain immobile (i.e., stationary); and the initial starting position (in rotating systems), is desirable.